Spinach (Spinacia oleracea) is a flowering vegetable plant in the family of the Amaranthaceae. It is native to southwestern and central Asia, but nowadays is being cultivated worldwide, mostly in temperate regions. The consumable parts of spinach are the leaves. These are produced during the first stage of the life cycle of a spinach plant, during which the plant forms a leaf rosette. The second stage is the flowering stage or bolting stage. Bolting is the growth of an elongated stalk with flowers grown from within the main stem of a plant. During the bolting stage it is not possible anymore to harvest any marketable product of the plant.
The leaves of a spinach plant are usually sold loose, bunched, in prepackaged bags, canned, or frozen. There are three basic types of spinach, namely savoy, semi-savoy and smooth. Savoy has dark green, crinkly and curly leaves. Flat or smooth leaf spinach has broad smooth leaves. Semi-savoy is a hybrid variety with slightly crinkled leaves.
Downy mildew is probably the most widespread and potentially destructive global disease of spinach. The causal agent of downy mildew disease on various plants of Chenopodiaceae, including spinach, is regarded as a single species, Peronospora farinosa. Some of the forms on important crop plants have been given names as formae speciales, so f. sp. spinaciae on spinach. Downy mildew affects the harvested part (leaves) of spinach (Spinacia oleracea). Yellow lesions appear on the older leaves. Although some fungicide treatments are effective, they are costly and cause ecological pollution. There is a need for resistant spinach cultivars. There are currently 13 recognised described P. farinosa races (races pf1 to pf13) of Peronospora farinosa f. sp. spinaciae and a number of isolates not officially recognized yet. Although the pathogen was first reported early in the eighteenth century, only three races of the pathogen had been identified before 1990. More than ten new races of the pathogen were identified between 1990 and 2010, and some of the newer races had overcome all known genetic resistance. The rapid ascendance of new races is likely to be a result of intensification and scaling-up in spinach production during the past decade.
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